This invention relates generally to energy transfer devices such as solid oxide fuel cells and is particularly directed to a monolithic solid oxide fuel cell having a first plurality of spaced linear channels for directing the flow of a gas fuel and a second plurality of spaced linear channels for directing the flow of an oxidizing gas for producing an output voltage when the gas-bearing fuel cell is operated at high temperatures.
A fuel cell is basically a galvanic energy conversion device that chemically combines hydrogen or a hydrocarbon fuel and an oxidant within catalytic confines to produce a DC electrical output. In one form of fuel cell, cathode material defines the passageways for the oxidant and anode material defines the passageways for the fuel, and an electrolyte separates the cathode and anode materials. The fuel and oxidant, typically as gases, are then continuously passed through the cell passageways separated from one another, and unused fuel and oxidant discharged from the fuel cell generally also remove the reaction products and heat generated in the cell. Being infeeds, the fuel and oxidant are typically not considered an integral part of the fuel cell itself.
The type of fuel cell for which this invention has direct applicability is known as the solid electrolyte or solid oxide fuel cell, where the electrolyte is in solid form in the fuel cell. In the solid oxide fuel cell, hydrogen or a hydrocarbon is used as the fuel and oxygen or air is used as the oxidant, and the operating temperature of the fuel cell is between 700.degree. and 1100.degree. C.
The hydrogen reaction on the anode (the negative electrode) with oxide ions generates water with the release of electrons; and the oxygen reaction on the cathode with the electrons effectively forms the oxide ions. Electrons flow from the cathode to the anode. Thus, the reactions are, at the: EQU cathode 1/2O.sub.2 +2e.sup.- .fwdarw.O.sup.-2 ( 1) EQU anode H.sub.2 +O.sup.-2 .fwdarw.H.sub.2 O+2e.sup.-. (2)
The overall cell reaction is EQU H.sub.2 +1/2O.sub.2 .fwdarw.H.sub.2 O (3)
In addition to hydrogen, the fuel can be derived from a hydrocarbon such as methane (CH.sub.4) reformed by exposure to steam at 350.degree. C. or above, which initially produces carbon monoxide (CO) and three molecules of hydrogen. As hydrogen is consumed, a shift in reaction occurs to EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2. (4)
The overall reaction of hydrocarbons in the cell is illustrated by EQU CH.sub.4 +2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O (5)
Inasmuch as the conversion is electrochemical, the thermal limitations of the Carnot cycle are circumvented; therefore efficiencies in the range exceeding 50% fuel heat energy conversion to electrical output can be theoretically obtained. This is much higher than equivalent thermal engines utilizing the same fuel conversion, including even a conventional diesel powered engine.
The electrolyte isolates the fuel and oxidant gases from one another while providing a medium allowing the ionic transfer and voltage buildup across the electrolyte. The electrodes (cathode and anode) provide paths for the internal movement of electrical current within the fuel cell to the cell terminals, which are connected to an external load. The operating voltage across each cell is of the order of 0.7 volts, so the individual cells must be placed in electrical series to obtain a useful load voltage. A series connection is accomplished between adjacent cells with an interconnect material which isolates the fuel and oxidant gases from one another while electrically coupling the anode of one cell to the cathode of an adjoining cell. As the active electrochemical generation of electricity takes place only across the electrolyte portions of the fuel cell, any interconnect separation between the cathode and anode in order to provide the series electrical connection between the cells renders that part of the fuel cell electrically nonproductive. The percentage of interconnect to electrolyte wall area defining each cell, if high, could significantly reduce the energy or power densities of such a fuel cell.
Diffusion of the reacting species (fuel or oxidant) through the electrodes to the electrolyte also limits the cell performance. Fuel and oxidant must diffuse away from the flow in the respective passageways through the electrolyte to the reaction sites. The fuel and oxidant diffuse through the electrodes to the electrolyte and react at (or near) the three-phase boundary of the gases, the electrodes (anode or cathode), and electrolyte, where electrochemical conversion occurs. As the hydrogen partial pressure of the fuel gases decreases along the length of the fuel passageways, less voltage is generated near or at the downstream end of the fuel passageways.
While it is possible to thermally and electrically extract great quantities of energy from the fuel, it is also inherently inefficient to extract such energies to the complete depletion of the fuel and oxidant. Complete conversion of the fuel in the fuel cell is thus not sought as it is intrinsically inefficient in the overall output of the cell voltage. For both a single cell and cells in gas flow series, the maximum theoretical voltage decreases along the cell. Practical fuel cells therefore consume only 80 to 90% of the fuel because the cell voltage decreases rapidly as the hydrogen becomes less than 5% of the fuel gas. The reduction in maximum cell voltage as the fuel is consumed is an important limitation.
One proposed series of solid oxide fuel cells utilizes a ceramic support tube, and the electrodes (anode and cathode) and electrolyte are built up as layers on the support tube. The support tube is confined in a sealed housing, and the fuel and oxidant are manifolded to the housing and the reaction products are ported from the housing as required. Depending on the layer build-up, the fuel is either conveyed internally of the support tube and the oxidant is conveyed externally of the support tube (or vice versa). A practical fuel cell unit would be composed of many such tubes supported within an exterior housing, and manifolding would separate and direct the fuel and oxidant proximate the tubes.
A typical support tube might be formed of calcia stabilized zirconia (ZrO.sub.2 +CaO); the cathode typically would be applied to the exterior face of the support tube and might be in the form of lanthanum manganite (LaMnO.sub.3); the electrolyte would be layered over a portion of the cathode, comprised, for example, of yttria-stabilized zirconia (ZrO.sub.2 +Y.sub.2 O.sub.3); and the anode would be layered over the electrolyte comprised, for example, of a nickel or cobalt yttria-stabilized zirconia cermet or mixture (Ni,Co+ZrO.sub.2 +Y.sub.2 O.sub.3). The oxidant would thereby flow internally of the structural tube while fuel would be circulated externally of the tube. For part of the cell where a series connection was to be made with an adjacent cell, the interconnection would be layered over the cathode at this location instead of the electrolyte and anode, to engage the anode of the adjacent cell. The interconnect might be comprised, for example, of lanthanum chromite (LaCrO.sub.3).
To form this type of fuel cell, the support tube must be formed with a high degree of porosity. Even with 40% porosity, the layered anode and cathode represent large diffusion barriers. The diffusion losses increase very steeply at high current densities and represent a limit on current and hence power. The minimum size of the support tube has been about 1 cm in diameter, with a side wall about 1 mm thick. A limiting factor of this support tube core arrangement is the length of path that the current must pass along the cathode and anode materials thereby inducing significant electrical resistant losses. In one effort to minimize this, the respective tubes have been shortened lengthwise and stacked end-to-end on one another, and the anodes and cathodes of the successive respective tubes have been interconnected in a serial fashion with an interconnect. This renders a single tube through which the fuel and/or oxidant passes, while the serial connection produces a higher voltage cumulative of the total number of serially interconnected individual tubes. The current flow is in line with the direction of the fuel and/or oxidant flow, namely axially of the tube configuration.
An alternate construction provides an electrical interconnect at a cordal arc section of the tube connected to the interior anode, for example, whereby adjacent tubes are stacked tangentially adjacent one another to establish a cathode-anode serial arrangement. As the current must pass circumferentially along the cathode and anode materials, significant electrical resistance losses are incurred. Moreover, the tube supports are nonproductive and heavy so that the power and energy densities suffer when compared to other forms of energy conversion, including even the liquid electrolyte fuel cells more commonly operated at lower temperatures.
In many prior designs of solid oxide fuel cells, a feed tube for the oxidant gas must fit witin the oxidant passageway thus requiring that the passageway be sufficiently large to receive the tube. The power density of the fuel cells is related to the cell size, so that reduction in cell size provides for higher power density. Certain prior designs also proposed complicated porting or manifolding of the cells, involving overlays of the materials that require great care in manufacture due to possible warping in the green state of the material layers which, in turn, also generally requires oversizing of the unit to provide for some margin of error should warpage occur and because of the inability to inspect and correct for this shortcoming.
One prior art approach is disclosed in U.S. Pat. No. 4,476,198 entitled "Solid Oxide Fuel Cell Having Monolithic Core", having John P. Ackerman and John E. Young as joint inventors. This patent discloses a monolithically formed core consisting only of materials active in the electrochemical reactions. This means that the electrolyte and interconnect walls of the core would be formed, respectively, only of anode and cathode materials layered on the opposite sides of electrolyte material or on the opposite sides of interconnect material. This allows the use of very thin material layers and very thin resulting composite core walls. The thin composite core walls can be shaped to define small passageways, while yet having sufficient structural integrity to withstand the fluid pressures generated by gas flow through the passageways and the mechanical stresses due to the weight of the stacked core walls on one another. This beneficially increases the power density of the fuel cell because of its reduced size and weight.
U.S. Pat. No. 4,476,197 entitled "Integral Manifolding Structure For Fuel Cell Core Having Parallel Gas Flow", having Joseph E. Herceg as sole inventor, discloses means for directing the fuel and oxidant gases to parallel flow passageways in the core. A core wall projects beyond the open ends of the defined core passageways and is disposed approximately midway between and parallel to the adjacent overlying and underlying interconnect walls to define manifold chambers therebetween on opposite sides of the wall. Each electrolyte wall defining the flow passageways is shaped to blend into an be connected to this wall in order to redirect the corresponding fuel and oxidant passageways to the respective manifold chambers either above or below this intermediate wall. Inlet and outlet connections are made to these separate manifold chambers, respectively, for carrying the fuel and oxidant gases to the core, and for carrying their reaction products away from the core.
U.S. Pat. No. 4,476,196 entitled "Solid Oxide Fuel Cell Having Monolithic Cross Flow Core and Manifolding", having Roger B. Poeppel and Joseph T. Dusek as joint inventors, discloses a monolithic core construction having the flow passageways for the fuel and for the oxidant gases extended transverse to one another, whereby full face core manifolding can be achieved for these gases and their reaction products. The core construction provides that only anode material surround each fuel passageway and only cathode material surround each oxidant passageway, each anode and each cathode material further being sandwiched at spaced opposing sides between electrolyte and interconnect materials. These composite anode and cathode wall structures are further alternately stacked on one another (with the separating electrolyte or interconnect material typically being a single common layer) whereby the fuel and oxidant passageways are disposed transverse to one another.
The U.S. Pat. No. 4,510,212 filed Oct. 12, 1983 entitled "Solid Oxide Fuel Cell Having Compound Cross Flow Gas Patterns", having Anthony V. Fraioli as sole inventor, discloses a core construction having both parallel and cross flow paths for the fuel and the oxidant gases. Each interconnect wall of the cell is formed as a sheet of inert support material having therein spaced small plugs of interconnect material, the cathode and anode materials being formed as layers on opposite sides of each sheet and being electrically contacted together by the plugs of the interconnect material. Each interconnect wall in a wavy shape is connected along spaced generally parallel line-like contact areas between corresponding spaced pairs of generally parallel electrolyte walls, operable to define one tier of generally parallel flow passageways for the fuel and oxidant gases. Alternate tiers are arranged to have the passageways disposed normal to one another. This provides for the solid mechanical connection of the interconnect walls of adjacent tiers to the opposite sides of the common electrolyte wall therebetween only at spaced point-like contact areas, where the previously mentioned line-like contact areas cross one another. The inert support material comprises between 2 and 98 wt. % of the whole core, varied as needed to minimize differential thermal expansion of the composite core wall structures.
The present invention offers various advantages not available in any of the prior art solid oxide fuel cells discussed above. For example, the solid oxide fuel cell of the present invention is comprised of only three components: (1) a pair of flat sheets; and (2) a corrugated sheet positioned between and intimately diffusion bonded with the two flat sheets and forming an integral structure having a plurality of parallel, elongated channels therewith. The corrugated structure of the channel-forming portion of the solid oxide fuel cell provides high strength to withstand the fluid pressures generated by gas flow through the passageways as well as mechanical stresses arising from the high operating temperatures of the fuel cell and the weight of the core walls on one another when a plurality of fuel cells are arranged in a stacked array. This corrugated channel-forming structure, because of its high strength per unit volume, also allows the composite core walls to be extremely thin in reducing the conduction path of the oxygen ions through the electrolyte and thus reducing fuel cell resistance. In addition, not only is the center portion of the fuel cell comprised of active regions, but the fuel intake and outlet manifolds of the cell are also active regions comprised of electrode material and thus allow for electrochemical reactions along the entire fuel cell length for increased energy conversion efficiency. In this manner, the maximum theoretical voltage is maintained substantially along the entire length of the fuel cell which further enhances fuel consumption efficiency. The present invention thus provides a solid oxide fuel cell with a monolithic core which has high structural integrity, is comprised of only three components and thus is easily fabricated and assembled, is easily insulated because of its small size, and also because of its small size and reduced weight provides increased power density.